U.S. Pat. No. 8,331,869 describes a system and method for recreating any desired near-field RF environment such that the field arriving at a device under test appears to have arrived from a radiating far field. This approach is now commonly known as an anechoic boundary array method, due to the use of an anechoic chamber to isolate the simulated environment inside the chamber and due to the antennas arrayed about the central volume used to create the RF boundary conditions necessary to produce the desired near field condition.
While both passive and active realizations of the anechoic boundary array method have been used, the common approach uses advanced RF channel emulators to emulate spatial channel models in order to simulate the multipath environment outside the boundary array. These RF emulators were designed to be used for cabled testing of wireless radios and typically do not provide the output power or receiver sensitivity needed for radiated testing. In order to condition signals for testing over the air, amplifiers are used between the spatial channel emulators and the antennas of the boundary array, providing additional power and gain on downlink or uplink signals.
FIG. 1 illustrates a typical anechoic downlink-only boundary array configuration 10 wherein the antennas 12 and device under test (DUT) 14 are within an anechoic chamber shown encompassed by dashed lines. An object of a test of the DUT 14 may be to measure the performance of the antenna system of the DUT 14 in an environment that simulates multipath propagation. Thus, the DUT 14 may be, for example, a mobile phone for which over the air performance in a multipath environment is to be evaluated. The DUT 14 may be a device capable of multiple input multiple output (MIMO) reception and transmission, adaptive beam forming, and/or antenna diversity.
To test the DUT 14, a base station emulator 16 generates complex signals that emulate signals from a base station such as an eNodeB in a long term evolution (LTE) wireless communication network. The base station emulator 16 may also introduce interference signals. The signals from the base station emulator 16 are fed to spatial channel emulators 18a and 18b, referred to herein collectively as spatial channel emulators 18. The spatial channel emulators 18 introduce propagation and fading to the signals from the base station emulator, to emulate multipath propagation as would be experienced in a real world environment. Interfering signals may also be introduced within the channel emulator to simulate interference from other devices elsewhere in the environment.
The outputs of the spatial channel emulators 18 are fed to power amplifiers 20 which amplify the signals. Cables conduct the amplified signals from the power amplifiers to antennas 12 surrounding the DUT 14. The antennas 12 may be capable of dual polarization and transmission from any desired azimuth and elevation about the DUT 14. At least one uplink communication antenna 22 receives signals transmitted from the DUT 14 and transmits them to the base station emulator 16.
Alternate implementations of the system include an uplink only configuration, where the directions of propagation and orientation of active system components such as amplifiers and channel emulators are reversed; and bi-directional implementations where both uplink and downlink signal components are fed to/from the boundary array using either the same or different antennas.
Early implementations of the anechoic boundary array were designed with bypass switches which allowed routing the active and passive paths of the system to a centralized location. This provided options for calibrating the individual components, such as the power amplifiers 20, as well as for using the antennas of the boundary array for traditional passive antenna pattern measurements (APM) and over-the-air (OTA) performance testing of radiated power and sensitivity, with the switch array allowing for high speed changes in propagation direction in lieu of mechanical positioning of the measurement antenna or DUT.
An example of such a system is shown in FIG. 2, where different test equipment 17 can be routed through first switches 19 to the spatial channel emulator 18 and boundary array. In addition those same signals can be routed to bypass the spatial channel emulator 18 and amplifier 20 using the switches 19 to directly access the array antenna 12 via switches 21. Likewise, switches 21 may be used to route the output of the spatial channel emulator 18 and amplifier 20 back to test equipment 17 in order to measure path losses along the conducted paths. A switch 23 may also be used to route the output of the spatial channel emulator 18 to a device under test for conducted testing. Note that for simplicity return paths are not illustrated. A given instrument will have both input and output connections routed simultaneously along different routes of the possible paths offered by the switches 19, 21 and 23.
Unfortunately, the approach of using bypass switches to calibrate sub paths of the system suffers from a number of shortcomings, including the problem that static paths in the system must be independently calibrated to remove their RF impact from that of the desired path between the test equipment and the center of the test volume. Also, the use of switches to change the path between the measurement path and the calibration path results in changes in the electrical lengths of intervening cables, which in turn, results in a different standing wave contribution between the measurement path and the calibration path.
Traditional range calibration techniques for APM and OTA testing disfavor the use of component testing in favor of end-to-end calibrations. Hence, over time, techniques for performing full end-to-end calibrations of the boundary array were developed which eliminated the need for manual or automated component testing of the individual active and passive system components.
FIG. 3 shows a single path of the MIMO system of FIG. 1. The signal routing 24, 26a and 26b enable connections to different communication test equipment as well as to different antennas within the anechoic chamber. Routes for performing conducted testing of radios (not pictured) using the same channel emulator 18 are commonly employed, also. By measuring the total path loss of the system with a constant tap channel model with known loss terms in the channel emulator, the losses of the various components external to the channel emulator, such as the signal routing 24 and 25 and the power amplifier 20, can be determined. These losses may then be applied to the internal losses associated with any other channel model in order to determine the power correction needed for device testing.
FIG. 4 illustrates a typical end-to-end calibration process of the path of FIG. 3, where a reference antenna 28 with a known gain relative to an isotropic radiator/receiver is placed in the test volume and used to determine the net path loss. The additional path loss components due to transmit and receive cables and connectors, as well as the reference antenna gain and internal losses of the vector network analyzer 30 or other test equipment, must all be applied as corrections to the measurement in order to determine the desired total path loss. Alternately, the signal routing connections may be used to measure sub-components of the total path, given the necessary corrections for the additional path loss components involved in the different signal routings. While automating switching for this purpose provides for relatively quick measurements of the various paths, this approach is generally inconvenient to set up since each of the added loss terms, including the relative differences through the switches for the calibration signal path vs. the path to the test volume, must be determined independently (with the associated measurement uncertainty) and then assumed to never change throughout the life of the system.
Despite the elimination of the need for calibration of individual components by using and end-to-end calibration, the overall complexity of the boundary array system does point to the necessity of a mechanism for monitoring the performance of the system on a regular basis. Evaluation of the total field produced in the center of the test volume is a first order approach to determining system accuracy, but can't detect degradation in a single system component from the aggregate sum of all of the signals within the chamber. It also generally requires additional process steps by the user to place a reference antenna within the test volume and perform the power validation. Given the capabilities of a typical channel emulator, it is possible to utilize a probing antenna somewhere outside the test volume to iteratively evaluate the output of each element in the array, at the cost of the additional test time involved in enabling and disabling each output of the channel emulation. By referencing the result to a reference measurement performed after the original range calibration, it is then possible to perform a non-intrusive evaluation of any drift in the system components, although at the cost of additional test time.